ARM: BCM53573: Initial support for Broadcom BCM53573 SoCs

BCM53573 series is a new family with embedded wireless. By marketing
people it's sometimes called Northstar but it uses different CPU and has
different architecture so we need a new symbol for it.
Fortunately it shares some peripherals with other iProc based SoCs so we
will be able to reuse some drivers/bindings.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Jon Mason <jon.mason@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>